Although nitric oxide (NO) is a central mediator during endotoxin-induced sepsis, direct detection of tissue NO in vivo, has until recently been difficult, and techniques have relied on indirect measurement of byproducts in blood or invasive technology. We have utilized electron paramagnetic resonance (EPR) in conjunction with the spin-trapping technique to detect NO directly, and non-invasively, from the tissue of septic mice. Relative signal intensity arising from NO complexed with iron and diethyldithiocarbamate (DETC) measured directly from the liver and kidney of mice given endotoxin was maximal at 6 hours post endotoxin. The quality of the EPR signal obtained (high signal to noise ratio of 15:1) using our experimental set-up and L-band EPR hardware was such that we were able to establish a time course of NO production in tissue following endotoxin, and measurement of NO from other organs (kidney and spleen). This technique was extended to experiments in which we first implanted an oxygen sensitive material (gloxy) into the liver of mice, and then monitored NO production following endotoxin. Due to the fact that the EPR spectrum from gloxy and that of NO-Fe-(DETC)2 do not overlap, we were able to monitor NO production and pO2 simulatneously in tissue, in real time.